1. Field of the Invention
The present invention relates generally to a drive apparatus, a lens unit, and a camera. More particularly, the present invention relates to a drive apparatus for driving a driven member by means of an oscillator using a piezoelectric element, and to a lens unit employing a lens drive mechanism for driving a lens using such a drive apparatus where the lens unit is used in a film camera, digital camera, video camera, microscope, binoculars, or other such products. The invention also relates to a camera, such as any of those identified above, employing a lens drive mechanism for driving a lens by means of the above-identified drive apparatus.
2. Description of the Related Art
Devices for driving movable parts in cameras and other such precision products normally use the drive force produced by an electromagnetic motor. See, for example, Japanese Unexamined Patent Appl. Pub. H10-161001, pages 3 to 4. The lens drive apparatus as taught in H10-161001 connects a plurality of lens groups, which are driven members, located inside the lens unit, to an electromagnetic motor disposed outside the lens unit by means of gears and cam rings. The gears and cam rings rotate in conjunction with the rotation of the drive shaft of the motor, and this rotation causes the lens groups to travel in an out along the optical axis for zooming and focusing.
The problem with such an electromagnetic-motor-driven drive apparatus is that, while the drive apparatus produces high drive power, the electromagnetic motor itself is relatively large and must be disposed externally to the lens unit, thus resulting in a large lens unit.
To reduce the size of the lens unit, devices that use the deformation of a piezoelectric element to drive the lens have also been proposed. See, for example, Japanese Unexamined Patent Appl. Pub. H7-274546 (pages 3 to 4), Japanese Unexamined Patent Appl. Pub. H8-66068 (pages 3 to 4), and Japanese Unexamined Patent Appl. Pub. H4-69070 (pages 3 to 5). A piezoelectric-element-based drive apparatus as taught in H7-274546, H8-66068, and H4-69070 has a lens and a lens frame as the driven member, a drive shaft frictionally coupled to the lens frame, and a piezoelectric element to which this drive shaft is fixed. When a voltage with a specified waveform is applied to the piezoelectric element, it oscillates by expanding and contracting along the drive shaft. This linear oscillation is transferred to the drive shaft, and the driven member, which is friction-coupled to the draft shaft, is thus driven. The voltage applied to the piezoelectric element is more specifically a pulse wave that causes the piezoelectric element to displace gradually in the drive direction and then displace rapidly in the opposite direction. The driven member moves in the drive direction as a result of friction with the drive shaft, but because the inertia of the driven member exceeds the friction when the piezoelectric element moves in the opposite direction, the driven member does not move in the opposite direction and is thus driven in the specified drive direction.
A large electromagnetic motor disposed externally to the lens unit, as is required with the drive apparatus taught in H10-161001, is therefore not required with the drive apparatus taught in H7-274546, H8-66068, and H4-69070, and a complicated mechanism having gears and cam rings for transferring the drive power of the electromagnetic motor to the driven member is not needed. The drive apparatus taught in H7-274546, H8-66068, and H4-69070 therefore affords a smaller lens unit with a simple construction.
With the drive method taught in H7-274546, H8-66068, and H4-69070, the expansion and contraction of the piezoelectric element when a voltage is applied is transferred directly to the drive shaft, and the driven member is driven by a friction coupling between the drive shaft and the lens frame. To drive the driven member quickly with this arrangement, the drive shaft must move a long distance with each oscillation, or more specifically the expansion/contraction displacement of the piezoelectric element must be great. This increases the size of the device in the expansion/contraction direction of the piezoelectric element.
Furthermore, while the driven member is held in the drive direction by friction with the drive shaft, the driven member can move easily and is unstable if this friction is too weak. Conversely, if friction is too great, energy loss when the shaft is driven increases, and drive efficiency deteriorates.